Formation of Anorthosite-Gabbro Rhythmic Phase Layering: An Example at North Arm Mountain, Bay of Islands Ophiolite

Rhythmically layered anorthosite and gabbro are exposed in a4–10-m thick interval at the base of the layered gabbrounit on North Arm Mountain, one of four massifs that composethe Bay of Islands ophiolite, Newfoundland. Within the rhythmicallylayered interval, up to 37 anorthosite layers 1–2 cm thickalternate with gabbroic layers 7–10 cm thick. Anorthositesare adcumulates (most contain <6ppm Zr) with 98–99%plagioclase (Plag) and 1–2% intergranular clinopyroxene(Cpx), whereas gabbros are adcumulates to mesocumulates (<6–20ppmZr) with 35–55% Plag, and the balance olivine (Ol) + Cpx? orthopyroxene (Opx). Average mineral compositions are: Olmg-number 100 ?Mg/(Mg + Fe)]=84?9, NiO=0?13wt. % Plag An =87?9; Cpx mg-number = 88?3, TiO₂=0?20 wt %; and Opx mg-number= 85?7. Rare earth element (REE) concentrations in clinopyroxeneand plagioclase are low throughout the rhythmically layeredinterval (<5 times chondrites). The rhythmically layeredinterval is sandwiched between thick layers of adcumulate toorthocumulate uniform gabbro with average modal proportionsof 54% Plag-39% Cpx-3% Ol-4% Opx. Average mineral compositionsare: Ol mg-number = 75?5, NiO = 0?08 wt. %; Plag An=69%6; Cpxmg-number = 81?2, TiO₂ =0?53 wt. %, and Opx mg-number = 77?5.Clinopyroxene and plagioclase REE abundances are systematicallyhigher in the uniform gabbro interval than in the rhythmicallylayered interval. Calculated fractional crystallization pathsand correlated cryptic variation patterns suggest that uniformand rhythmically layered gabbros represent 20–30% in situcrystallization of two distinct magma batches, one more evolvedand the other more primitive. When the more primitive magmaentered the crystallization site of the NA300–301 gabbros,it is estimated to have been 40?C hotter than the resident evolvedmagma, and may have been chilled by contact with a magma chambermargin composed of uniform gabbro. In this model, chilling causedthe liquid to become supercooled with respect to plagioclasenucleation temperatures, resulting in crystallization of gabbrodeficient in plagioclase relative to equilibrium cotectic proportions.Subtraction of a plagioclase-poor melagabbro enriched the liquidin normative plagioclase, which in turn led to crystallizationof an anorthosite layer. Alternating anorthosite and gabbrolayers in the rhythmically layered interval built up by coupledand sustained variations in crystal nucleation and growth rates,and associated variations in liquid compositions at the crystallizationfront. Relatively stagnant magma-flow conditions may be requiredto accumulate substantial thicknesses of rhythmically layeredcumulates by sustained oscillatory crystallization. The rarityof anorthosite-gabbro rhythmic phase layering on North Arm Mountainmay indicate that convective magma currents in the Bay of Islandsmagma chamber were too vigorous for oscillatory crystallizationto commonly occur.